Syntheses and properties of thienyl-substituted dithienophenazines

Article abstract of DOI:10.3762/bjoc.6.135

A series of dithienophenazines with different lengths of the oligomeric thiophene units (quaterthiophenes and sexithiophenes) was synthesized. The thiophene and phenazine units act as electron donors and acceptors, respectively, resulting in characteristic absorption spectra. The optical spectra were calculated using time-dependent density functional theory at the B3LYP/TZVP level and verify the experimental data. Adsorption of the dithienophenazines on highly ordered pyrolytic graphite (HOPG) was investigated by scanning tunneling microscopy, showing that one of the compounds forms highly organized self-assembled monolayers.

Full text of DOI:10.3762/bjoc.6.135

Syntheses and properties of thienyl-substituted
dithienophenazines
Annemarie Meyer1,§, Eva Sigmund1,§, Friedhelm Luppertz1,
Gregor Schnakenburg2, Immanuel Gadaczek3, Thomas Bredow*3,
Stefan-S. Jester1 and Sigurd Höger*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2010, 6, 1180–1187.
1Kekulé-Institut für Organische Chemie und Biochemie, Rheinische
doi:10.3762/bjoc.6.135
Friedrich-Wilhelms-Universität Bonn, Gerhard-Domagk-Str. 1, 53121
Bonn, Germany, 2Institut für Anorganische Chemie, Rheinische
Friedrich-Wilhelms-Universität Bonn, Gerhard-Domagk-Str. 1, 53121
Bonn, Germany and 3Institut für Physikalische und Theoretische
Chemie, Rheinische Friedrich-Wilhelms-Universität Bonn, Wegelerstr.
Received: 27 July 2010
Accepted: 08 November 2010
Published: 13 December 2010
12, 53115 Bonn, Germany
Associate Editor: P. J. Skabara
Email:
© 2010 Meyer et al; licensee Beilstein-Institut.
License and terms: see end of document.
Thomas Bredow* - bredow@thch.uni-bonn.de;
Sigurd Höger* - hoeger@uni-bonn.de
*
Corresponding author
§
Both authors contributed equally to this publication
Keywords:
oligothiophenes; phenazines; scanning tunneling microscopy;
self-assembled monolayers; TD-DFT calculations
Abstract
A series of dithienophenazines with different lengths of the oligomeric thiophene units (quaterthiophenes and sexithiophenes) was
synthesized. The thiophene and phenazine units act as electron donors and acceptors, respectively, resulting in characteristic
absorption spectra. The optical spectra were calculated using time-dependent density functional theory at the B3LYP/TZVP level
and verify the experimental data. Adsorption of the dithienophenazines on highly ordered pyrolytic graphite (HOPG) was investi-
gated by scanning tunneling microscopy, showing that one of the compounds forms highly organized self-assembled monolayers.
Introduction
Thiophene based oligomers and polymers have drawn consider- deposition from either vacuum or solution (casting or printing),
able interest as active materials in various fields of organic elec- and thus they are already utilized in industrial manufacturing.
tronics such as organic light-emitting diodes (OLEDs), organic Particularly, deposition from the solution phase requires suffi-
thin-film transistors (OTFTs), or organic photovoltaics (OPVs) ciently high solubility, and one approach to cover this aspect is
[
1-3]. Concerning applications, organic materials are highly the functionalization with long, flexible alkyl substituents.
attractive due to their low cost and their particularly simple Additionally, in many cases (short) oligomers are considerably
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Beilstein J. Org. Chem. 2010, 6, 1180–1187.
better soluble than their corresponding (long) polymers, and
can, as a matter of principle, be obtained in higher purity with
respect to defects/polymerization faults. Moreover, they act as
model systems allowing estimation of the pure polymer prop-
erties by extrapolation of the oligomer properties to infinite
molecular weight [4].
Beside pure thiophene-based (well-established) materials, com-
pounds with additional electron-poor moieties are in focus, as
they are known to shift the HOMO and LUMO levels towards
lower energies, thus increasing the compounds' stability against
oxidation. In addition, the HOMO-LUMO gap is reduced (by
the donor-acceptor (DA) approach), thereby red-shifting the
absorption edge. This is of special importance, as there is still a
need for new materials (exhibiting additionally the above
mentioned processability criteria) absorbing the longer wave-
length region (> 600 nm) of the sunlight spectrum, being
Scheme 1: Synthesis of 4. a) 1) EtLi, Et2O, −78 °C, 1 h; 2) (COOMe)2,
Et2O, −78 °C, 2 h, 33%; b) FeCl3, MeNO2, CH2Cl2, rt, 16 h, 71%; c)
Br2, AcOH, CHCl3, reflux, 5 h, 95%.
required for photocurrent generation also from respective low- thesis of 3 has previously been reported by benzoin conden-
energy photons and thus enhancing the overall light harvesting sation of the corresponding bithiophene derivative [18].
yield in organic photovoltaics (OPV). For example, in the field
of fused N- and S-heterocycles, thieno[3,4-b]pyrazines with Alternatively, we prepared first 3,3’-thenil (2) by metalation of
various side groups have already found applications in OPVs 3-bromothiophene and reaction with dimethyl oxalate [19].
[
5-8]. In addition, fused bithiophenes with an enforced Subsequent oxidative intramolecular thiophene-thiophene
planarity, and therefore reduced optical gaps, and an increased coupling [20] with FeCl3 yielded 3 as a dark red (nearly black)
π–π overlap of the polycyclic molecule backbone have entered solid. Bromination of 3 with bromine in acetic acid/chloroform
various fields of organic electronics [9-13].
gave 4 in nearly quantitative yield [21,22]. The respective regio
isomer 8 was synthesized starting from commercially available
Apart from their optoelectronic properties, linear and branched 3-bromothiophene 1 and boronic acid 5 in three steps
as well as cyclic and polycyclic thiophenes are valuable (Scheme 2). By Suzuki–Miyaura coupling we obtained 3,3’-
templates for the epitaxial coadsorption of adlayers, in particu- bithiophene in good yields. The red diketone 7 was prepared by
lar for fullerenes and metallacycles [14-17].
two-fold acylation with oxalyl chloride [23,24]. Bromination of
with NBS failed, however, the reaction with bromine under
similar conditions as described for 4 afforded 8 in quantitative
7
Results and Discussion
Here we report the syntheses of two isomeric benzodithiophene- yields. Both routes towards the brominated bithiophene-dike-
diones, their respective phenazines and their coupling products tones gave high overall yields and can be scaled up easily.
with thiophene boronic acids. In both cases the thiophene units
of the molecules act as electron rich parts (donors, D), whereas
the phenazine moieties serve as electron deficient parts (accep-
tors, A), leading to a bathochromic shift of the UV–vis spectra
in comparison with the non-condensed thiophene analogues.
These assignments of the thiophene moieties as donors and the
phenazine moieties as acceptors are confirmed by quantum-
chemical calculations at the density-functional level.
Additionally, one of the compounds forms a self-assembled
monolayer on a HOPG surface, as imaged by scanning
tunneling microscopy (STM).
Synthesis
Scheme 2: Synthesis of 8. a) Pd(PPh3)4, Na2CO3, toluene, EtOH,
H2O, reflux, 87%; b) oxalyl chloride, 1,2-dichloroethane, reflux, 85%; c)
Br2, CHCl3, AcOH, reflux, 10 h, 97%.
The synthesis of 4 is shown in Scheme 1. It is obtained in three
steps from commercially available starting materials. The syn-
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Beilstein J. Org. Chem. 2010, 6, 1180–1187.
Scheme 5: Suzuki–Miyaura reactions with 11a/11b. a) Pd(PPh3)4,
Na2CO3, toluene, EtOH, H2O, reflux, 50% (16), 15% (17a), 19% (17b);
b) Pd(PPh3)4, 2 M Na2CO3, toluene, Aliquat 336, reflux, 56% (18).
Scheme 3: Preparation of phenazine isomers. a) AcOH, 50 °C, 2 h,
3% (10a), 53% (10b). b) AcOH, 50 °C, 2 h, 63% (11a), 50% (11b).
9
Condensation of the diketones 4 and 8 with diaminobenzenes Nevertheless, both compounds can serve as starting materials
a and 9b, containing methyl and hexyl side groups [25], in for thiophene oligomers with increased donor ability. Thio-
9
pure acetic acid at 50 °C gave the phenazines 10 and 11 as phene- and bithiophene boronic acids or esters were coupled via
yellow solids in 50–93% yield (Scheme 3) [26]. The hexyl Suzuki–Miyaura reactions forming orange-colored thiophene-
derivatives 10b and 11b showed a significantly higher solu- tetramers 12a, 12b, 14 and 16 and red-colored hexamers 13, 15,
bility than the methyl analogues and could be more easily puri- 17a, 17b and 18, respectively (Scheme 4 and Scheme 5).
fied by column chromatography.
The low solubility of the methyl derivatives 12a, 16 and 17a
made their purification tedious. However, this problem did not
occur in the case of the hexyl substituted oligomers. Highly
soluble compounds (20–50 mg/mL in chloroform or dichloro-
benzene) were obtained by coupling of the hexyl substituted
compounds 10b and 11b with alkyl substituted thiophene or
bithiophene boronic acids (esters).
UV–Vis spectra
Figure 1a shows the UV–Vis absorption and emission spectra of
1
2a, 13, 14 and 15 in solution (CH2Cl2). In Figure 1b the
spectra of the isomeric compounds 16, 17a and 18 are displayed
27]. All oligomers based on 10 exhibit strong absorption bands
[
at ~250–350 nm and at ~350–450 nm with underlying vibronic
structures. As expected, the longest wavelength absorption is
for the hexathiophene derivatives shifted bathochromically in
comparison to the tetrathiophene derivatives. The spectra of 16,
1
7a and 18 display two strong bands at ~300–400 nm and at
~
400–500 nm, some of them show vibronic fine structures. The
Scheme 4: Suzuki–Miyaura reactions with 10a/10b. a) Pd(PPh3)4,
Na2CO3, toluene, EtOH, H2O, reflux, 23% (12a), 36% (12b), 26% (13);
b) Pd(PPh3)4, 1 M Cs2CO3, toluene, reflux, 19% (14), Pd(PPh3)4, 2 M
Na2CO3, toluene, Aliquat 336, reflux, 21% (15).
maxima of the emission spectra are shifted by ≈3500 to
4600 cm−1 with respect to the longest wavelength absorption
maxima (Figure 1, Table 1).
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Beilstein J. Org. Chem. 2010, 6, 1180–1187.
Figure 1: a) Absorption (solid lines) and emission (dotted lines) spectra of compounds 12a (blue), 13 (red), 14 (green) and 15 (black) in solution
(CH2Cl2). b) Absorption (solid lines) and emission (dotted lines) spectra of compounds 16 (blue), 17a (red), 18 (black) in solution (CH2Cl2).
Table 1: Spectral data of the synthesized oligomers. Absorption and emission spectra were measured in CH2Cl2 solutions. The zero–zero transition
E0–0 values were estimated from the intersection of the absorption and emission spectra.
λabs.max
λabs.onset
[nm]
λabs.onset
[eV]
λem.max
[nm]
Stokes shift
[cm−1]
E0–0
[nm]
E0–0
[eV]
[nm]
1
1
1
1
2a
440
462
491
495
513
550
561
577
2.41
2.25
2.17
2.13
519
583
606
622
3459
4522
3865
4125
463
524
531
556
2.68
2.37
2.33
2.23
4
3
5
1
1
1
6
465
478
476
500
530
539
2.48
2.34
2.30
525
585
610
2457
3833
4615
493
520
531
2.51
2.38
2.33
7a
8
At the onset of the absorption, the zero–zero transition and the Quantum chemical calculations
longest wavelength absorption maxima follow the same To rationalize the observed trends in the optical absorption
expected) trends, that is the more extended π-systems (13, 15, spectra of the oligomers, DFT calculations using the ORCA
7, 18) exhibit a smaller optical gap. Moreover, when compared program package [29] were carried out. The ground state
with the optical spectra of the quater- and sexithiophene [28] geometries of quaterthiophene 19, the compounds 12a, 16,
Table 2, 19 and 20), a clear red-shift could be observed for sexithiophene 20 and the compounds 13 and 17a were opti-
(
1
(
both oligomer series, regardless if they are based on 10 or 11, mized with the B3LYP functional [30] with a TZVP basis set
due to the intramolecular donor-acceptor character of the com- [31]. Different rotations of the thiophene rings were investi-
pounds.
gated each corresponding to a different local minimum-energy
Table 2: Spectral data of oligothiophenes in CH2Cl2..
λabs.max
λabs.onset
[nm]
λabs.onset
[eV]
λem.max
[nm]
E0–0
[nm]
E0–0
[eV]
[nm]
Quaterthiophene (19)
Sexithiophene (20)
393
435
450
505
2.75
2.45
455, 478
510, 537
440
488
2.82
2.54
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Beilstein J. Org. Chem. 2010, 6, 1180–1187.
structure. The differences in the total energy of the ground gies are in reasonable agreement with the experimental results
states are within the typical DFT error range of 3 to 11 kJ/mol. (see also Table 3). It has to be kept in mind that the calculated
It can therefore be expected that at room temperature all confor- values correspond to vertical excitation energies between elec-
mations are present. For simplicity we only present results for tronic states while the measured optical spectra include vibronic
the most stable structure. For the treatment of excited states effects.
time-dependent density functional theory (TD-DFT) was
applied. The COSMO model was used to simulate the solution The dominating signals in the spectra of quaterthiophene 19 and
environment of the molecules in CH2Cl2 [32,33].
sexithiophene 20 correspond to the HOMO-LUMO excitation.
While the calculated S1 excitation energy of 19 differs by about
The calculated spectra of the most stable conformers and the 0.4 eV from the experimental result, the excitation energy of 20
Sn–S0 difference electron densities of selected compounds are is within the typical error range of B3LYP (ΔEexp-calc = 0.1 eV)
shown in Figure 2. In general, the calculated excitation ener- [34]. Both compounds have a similar spectrum, but the transi-
Figure 2: Calculated absorption spectra of (a) 19, (b) 20, (c) 12a, (d) 13 with R=CH3, (e) 16 and (f) 17a. In every spectrum the ground state geometry
and the difference densities of the excitations are shown. Blue (red) color indicates a decrease (increase) of electron density after transition from the
ground state to the excited state. Absorption signals with large oscillator strengths are labeled with the main excitation inducing the transition to this
state.
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Beilstein J. Org. Chem. 2010, 6, 1180–1187.
Table 3: Lowest vertical excitation energies obtained by B3LYP/TZVP TD-DFT calculations of the quaterthiophene 19 and derivatives as well as
sexithiophene 20 and derivatives (gas phase and in CH2Cl2) in eV, and oscillator strengths fosc.
State
19
12a
16
Gas
CH2Cl2
fosc
Gas
CH2Cl2
fosc
Gas
CH2Cl2
fosc
S1
S2
3.14
4.80
3.13
4.78
1.39
0.13
2.37
3.18
2.11
3.18
0.19
1.02
2.69
3.53
2.56
3.49
0.26
1.17
State
20
13 (R=CH3)
17a
Gas
CH2Cl2
fosc
Gas
CH2Cl2
fosc
Gas
CH2Cl2
fosc
S1
S2
2.64
3.74
2.63
3.73
2.27
0.35
2.21
3.18
1.97
3.15
0.45
1.46
2.43
2.46
2.21
2.24
0.08
0.30
tions of 20 are red shifted due to its larger delocalized π-system, (SAMs) at the HOPG/TCB interface (highly oriented pyrolytic
which lowers the HOMO-LUMO gap as discussed above.
graphite/1,2,4-trichlorobenzene). In situ STM was applied
under typical conditions with typical tunneling parameters
The difference density of 12a (Figure 2c) shows that the S0-S1 normally allowing the visualization of SAMs – if formed [36].
excitation leads to an intramolecular charge transfer (CT) from However, among all the substances only 12b (10−5 M in TCB)
the thiophenes to the phenazine part of the molecule. These assembles into crystalline monolayers on HOPG (Figure 3a).
calculations verify our initial assumption that the electron rich Bright and dark colors originate from locally high and low
thiophene units act as electron donors and the electron poor tunneling currents, resulting from unsaturated (backbone) and
phenazine unit as an electron acceptor in this intramolecular saturated (alkyl side chain) hydrocarbon segments, respectively
CT-complex. This CT state can also be observed in 16 [37]. A unit cell with a = 2.5 ± 0.1 nm, b = 2.3 ± 0.1 nm,
(
Figure 2e), but here the calculated difference between the S1 γ = 90 ± 2° could be determined [38]. A proposed molecular
energies of 12a and 16 (0.27 eV) is larger than the measured model of the adsorbate geometry is shown in Figure 3b.
value (0.07 eV). Nevertheless, the computed excitation ener-
gies show a similar trend as the observed maxima of the absorp- The banana-shaped bright features in Figure 3a are attributed to
tion spectra.
the backbones of 12b, assembling in densely packed double
rows. However, short alkyl chains are commonly not visible
The increased length of the thiophene chains in 13 compared to under the applied conditions. Nevertheless we assume they are
1
2a lowers the excitation energies of the CT states by about oriented along the main crystallographic direction of the HOPG
0
.2 eV due to an extended electron delocalization, as it is also substrate.
experimentally observed. The same holds for the isomeric
series. The calculated and observed optical gap in 17a is consid- Ordered adlayers were observed only for 12b, but not for 12a.
erably smaller than in 16. It is worth noting that in both hexa- Moreover, additional terminal alkyl chains terminating the end-
thiophene derivatives the charge reorganization upon excitation capping thiophene substituents of 14, 15 and 18 appear to
includes also the outermost thiophene rings, indicating that an hinder the formation of a self-assembled monolayer, most prob-
extension of the chain would further reduce the optical gap. In ably due to sterical hindrance of the molecules to cover the
addition, 17a has a nearly degenerate S2 state, which also has surface densely.
CT properties. This HOMO-1 to LUMO excitation is visible in
1
7a but not in 13 since the orbital energy and shape of HOMO Conclusion
and HOMO-1 in 17a are nearly the same (ΔEHOMO,HOMO-1 We have developed a simple method for preparing phenazine-
0.02 eV), while in 13 the energy difference between HOMO thiophene oligomers. These compounds show interesting optical
=
and HOMO-1 is larger than 0.6 eV [35].
characteristics, which can be tuned by attachment of different
thiophene substituents. Currently we investigate the electro-
optical properties of these compounds and similar derivatives.
STM study
All synthesized thiophene oligomers 12–18 were investigated In addition, for one of the compounds a SAM on HOPG could
concerning their ability to form self-assembled monolayers be observed.
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Beilstein J. Org. Chem. 2010, 6, 1180–1187.
Figure 3: Self-assembled monolayer of 12b on HOPG. a) STM image (VS = −0.8 V, It = 80 pA, image size 25.0 × 25.0 nm2); b) structure model [39].
3
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MS data for all new compounds are supplied, including
X-ray data and a cif-file for 4 and 7.
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Supporting Information File 1
Syntheses and properties of thienyl-substituted
dithienophenazines, experimental procedures, and
characterization.
6
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doi:10.1002/ange.200702506
[http://www.beilstein-journals.org/bjoc/content/
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supplementary/1860-5397-6-135-S1.pdf]
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Yan, Q.; Zhou, Y.; Ni, B.-B.; Ma, Y.; Wang, J.; Pei, J.; Cao, Y.
J. Org. Chem. 2008, 73, 5328–5339. doi:10.1021/jo800606b
Supporting Information File 2
Syntheses and properties of thienyl-substituted
dithienophenazines – cif files.
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5.The calculated solvent shift is negligible for the parent compounds 19
and 20 (Table 2 and Table 3). Due to their CT character, the
bathochromic shift is more pronounced for the S1 states of the
substituted compounds 12a, 16, 13 and 17a.
3
3
3
6.A drop of the respective solution (approx. 1 mg/ml) in TCB
(1,2,4-trichlorobenzene) was applied to a piece of freshly cleaved
HOPG, and STM was performed at voltages between −(0.4–1.2V) and
currents between 2–100 pA.
7.The tunneling efficiency depends on the energetic difference of the tip
level and the levels of the frontier orbitals of the respective molecular
segments. For theoretical descriptions of the contrast mechanism in
STM see i.e. [40,41].
8.All acquired STM images were in situ calibrated by (immediately and)
subsequently reducing the tip bias to −50 mV, and acquiring the
atomically resolved lattice of the HOPG substrate (without changing the
position), using the latter as a calibration grid for the images of the
adlayer. Respective data handling is done with the SPIP software
package (Image Metrology, Inc.).
3
9.The resolution of STM is not sufficient to distinguish between cisoid
and transoid conformers of the oligothiophene. However, in the
molecular model we displayed the adsorbed molecule in transoid
conformation in accordance with the TD-DFT calculation.
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